1. Field of the Invention
This invention relates to illuminated signs. More particularly, it relates to signs comprising photoluminescent quantum dots (QDs).
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Illuminated Signage
Illuminated signage has applications in a wide variety of sectors, from road safety and warning or emergency signs to advertising boards and shop fronts. Illuminated signage may be made from a range of different lighting sources, and may comprise static or rolling displays. Conventional lighting displays traditionally utilize solid-state lighting. Color is an important aspect of signage, since it may be used to convey a message by association, e.g. red often signifies danger. The human eye is also more receptive to particular wavelengths of light than others; in normal light conditions the human eye is most sensitive to light around 555 nm, i.e. yellow-green, while in low-intensity light conditions the eye becomes more receptive to violet and blue light and less sensitive to green and red light. Thus, lighting systems that can provide a wide range of colors across the visible spectrum are advantageous.
Illuminated lighting may be static, flashing, or rolling, whereby a moving message is displayed. Particular lighting systems are often more suited to one display format than another, for instance liquid crystal displays, which have long switching times, are poorly suited for flashing signage. A sign may be “back-lit” whereby the illumination comes from behind the sign, “front-lit” where illumination is typically by swan neck lights shining on the front of the sign, or “edge-lit” where an opaque sign is indirectly illuminated by backlighting to give a halo effect.
Signage Applications
In many jurisdictions, legislation is in place having requirements for illuminated traffic and safety signage. For example, “The Traffic Signs Regulations and General Directions” legislation of 1994 stipulates that, in the UK, internally or externally illuminated signage is mandatory during the times that street lighting is in use or during the hours of darkness, on any road within 50 m of a lamp lit by electricity that acts as part of a system of street lighting. Exemptions apply for temporary signage; however this must be illuminated by retro-reflective material. Estimates from a US study suggest that replacing incandescent traffic signs with LEDs could reduce energy costs by 93%; with an installation cost estimated at $300 for replacing an incandescent bulb with LEDs, the annual energy saving calculated at 1,266 kWh could save $125 in energy [“Responsible Purchasing Guide: LED Exit Signs, Street Lights, and Traffic Signals”, Responsible Purchasing Network, 2009]. Failure of incandescent bulbs and fluorescent lighting can be instantaneous, which in traffic sign applications may have potentially serious consequences. Therefore, alternative signage for which the failure is gradual (e.g. dimming over time) is desirable since it offers warning, allowing time for the signage to be replaced.
The “Health and Safety” legislation of 1996 requires that, in the UK, the light emitted from illuminated signs must produce a luminous contrast that is appropriate to its environment, such that there is no excessive glare from an excessive amount of light, or poor visibility as a result of insufficient light. Specific colors must be adhered to; red for prohibition, danger, and fire-fighting equipment signs, yellow/amber for warning signs, blue for mandatory signs, and green for emergency escapes, first aid signs, and to signify no danger. As with traffic signs, failure may have potentially hazardous consequences, thus an illumination system that decays gradually rather than instantly is advantageous.
Illumination may be applied to advertising boards to attract the attention of the observer. Advertising displays benefit from a lighting system that is easily adaptable, since advertisements are often temporary, thus a permanent back-lit system combined with a temporary fascia is often favorable. If the fascia is temporary, then a low-cost but quick manufacturing method is desirable, while display lifetime is less important.
Illuminated shop/business front signs may be used to attract the attention of passers-by and to make an entrance more visible during the hours of darkness. This is particularly effective for businesses that predominantly operate at night, such as bars, restaurants and nightclubs. The lighting displays may be required in any color, and are usually illuminated continuously for extended periods of time, thus a display that is inexpensive to power is desirable. Shop/business front signs are often large in size, therefore a technology without size restrictions is preferable.
Information signs, e.g. exit, toilets, “please pay here”, etc. may be illuminated to enhance their visibility. Such signs are required in almost any color to suit the tastes and requirements of the consumer. The signs are likely to require continuous illumination for extended periods of time, therefore a reliable lighting system that is inexpensive to power is advantageous.
In total, illuminated signage makes considerable contribution to worldwide energy costs and CO2 emissions. By using more “green” illuminated signage technologies, such as the QD signage displays described herein, not only can energy and CO2 emissions be reduced, but also cost. With escalating energy costs, the initial investment cost of installation of QD signage displays can be recuperated by the energy savings, which for public funded signage could be favorable to the tax-payer. The current invention further provides a reliable illumination source, which decays gradually rather than failing instantly. The illuminated devices disclosed herein may be used to fabricate many different types of signage and are not restricted to the aforementioned applications.
Display Technologies
“Neon lighting” is often used to refer to gas discharge lighting tubes containing neon or other gases. The tubes contain a rarefied gas, across which a voltage is applied to liberate electrons from a tungsten cathode. The electrons collide, ionizing the gas inside the tube to form a plasma. Neon lighting was first exploited when it was realised that discharge from a neon-filled lamp produced vibrant red light. The term “neon lighting” has now come to encompass other gas discharge lamps, including argon, xenon, krypton, and mercury vapour. Phosphor coatings on the inside of the tube may be used to tune the emission, producing a wide array of colors. Phosphorescent materials emit at a longer wavelength than they absorb, as the absorbed radiation undergoes a Stokes shift. Examples of phosphors include BaMg2Al16O27:Eu2+ (450 nm blue emission), Zn2SiO4(Mn,Sb)2O3 (528 nm green emission), Mg4(F)(Ge,Sn)O6:Mn (658 nm red emission). In conventional neon lighting, when the lamp is switched on, the cathode is heated to its thermionic emission temperature, thus liberating electrons. A variation on this principle is cold-cathode lighting, in which electrons are liberated below the thermionic emission temperature. Consequently, cold-cathode tubes typically last longer than conventional neon lights, however they are less efficient. A further advantage is that they can be instantly switched on and off. Neon lighting can last for many years, however the tubes are susceptible to absorption of the gas by the glass walls of the tube, increasing the resistance of the tube such that it cannot be illuminated by the applied voltage. Further, there are issues surrounding the safety of neon lights; tubes can be under a partial vacuum, and thus if broken can implode. Toxic mercury vapor may be released. If a cut is sustained from phosphor-coated glass, the phosphor can prevent blood clotting. Since gas discharge lamps have high energy losses as heat, their use is limited to applications that are out of human reach to minimise the risk of burns through physical contact.
The use of light-emitting diodes (LEDs) in illuminated signage is becoming increasingly popular. LEDs are used both directly as the lighting source and indirectly as backlights in conjunction with color filters. LEDs are traditionally made from inorganic semiconductors, which emit at a specific wavelength, e.g. AlGaInP (red), GaP (green), ZnSe (blue). Other forms of solid-state LED lighting include organic light-emitting diodes (OLEDs), wherein the emissive layer is a conjugated organic molecule such that delocalised π electrons are able to conduct through the material, and polymer light emitting diodes (PLEDs), in which the organic molecule is a polymer. Advantages of SSL over traditional incandescent lighting include superior longevity, lower energy consumption resulting from less energy loss as heat, superior robustness, durability and reliability, and faster switching times. As little heat is dissipated, the bulbs are safe to touch, which is particularly advantageous for signage applications as it allows the sign to safely be cleaned and maintained during or shortly after illumination. However, SSL is expensive, and it is difficult to produce high quality white light. Several approaches to produce white light from solid-state LEDs have been explored. White light can be obtained by using three or more LEDs of differing wavelengths, e.g. with red, green and blue emission, producing high efficiency white light. However, this approach is very expensive and it is difficult to produce pure white light. Other approaches combine an LED emitting in the UV or blue region of the electromagnetic (EM) spectrum with a phosphor. One such approach is to use a combination of a UV or blue LED with a number of phosphors, e.g. a red and a green phosphor, such as SrSi:Eu2+ and SrGaS4:Eu2+, respectively. Alternatively, a blue LED and a yellow phosphor may be combined, producing a less expensive white light source, however the color control and color rendering index of such materials is usually poor, owing to the lack of tuneability of the LED and phosphor.
Lightboxes may be used for backlighting in illuminated signage. LED or fluorescent lighting may be employed. The face panel that contains the image may be made from translucent acrylic or flex-face materials. Flex-face material allows a sign of any size to be fabricated from a single piece of material, thus avoiding the challenges involved with joining adjacent acrylic panels. Lightboxes are advantageous for temporary signage, such as advertisements, as the fascia may be readily substituted without having to change the backlighting. However, the lighting is restricted to a single color of light.
Dot matrix signs are typically used to display messages, such as announcements on public transport. The sign consists of a matrix of lights, from LEDs, liquid crystals or cathode ray tubes. The lights can be switched on or off to display text and graphics, which may also be programmed to scroll across the display. Though dot matrix signs are relatively inexpensive, reliable and easy to read, they are usually restricted to single color displays so that the display may readily be changed.
Side-emitting fibre optic cables may be used as an alternative to neon lighting for signage applications. In fibre optics, light from an LED or laser source is transmitted along a glass fibre that consists of a transparent core enclosed in a lower refractive index cladding material to result in total internal reflection. For side-emitting cables, there is a rough interface between the core and the cladding material, such that not all light is totally internally reflected and a small amount is scattered. No heat or electricity is transmitted through the optical fibres, making them safe for outdoor use in all weather conditions, which is particularly advantageous for safety signage. Neither is there a risk of sparks from a broken fibre. Typical light sources include LEDs, quartz halogen lamps and xenon metal halide lamps. Drawbacks of fibre optics include high installation costs, and for side-emitting fibres the length of cable is limited due to the loss of light along the cable.
Lenticular displays, which may be illuminated, are used to produce an image that appears to move or change as it is viewed from different angles. Lenticular displays are particularly suited to advertising signage. Disadvantages of lenticular displays include their high production cost and the display thickness, which may be large due to the lenses required.
Plasma displays exploit a similar technology to discharge and fluorescent lighting. Millions of tiny cells are encased between two glass panels. The cells contain a mixture of noble gases and mercury. When a voltage is applied across a cell, the mercury vaporises and a plasma forms. As the electrons collide with the mercury atoms, UV light is emitted, which excites the phosphor coating on the inside of the cell to produce visible or infrared (IR) radiation. Approximately 60% of radiation is typically emitted in the IR. In a plasma display, each pixel consists of three cells: one emitting red, one emitting green and one emitting blue light. Different colors are produced by varying the voltage. Advantages of plasma displays for signage applications include that they have a wider viewing angle than other forms of display, such as liquid crystal displays (LCDs). Further, they have a slim profile. However, plasma display screens are relatively expensive to produce and operate, with higher energy consumption than LCDs and LEDs. They often suffer from “screen-door effects”, where the fine lines between pixels become visible. Plasma display signage is poorly suited for use at high altitudes, since the pressure differential between the air pressure and that of the gases inside the display can create a buzzing sound.
Electroluminescent (EL) displays are made from semiconducting material sandwiched between two conducting layers. The bottom layer is usually made from a reflective material, while the top layer is usually a transparent conductor, such as indium tin oxide, to transmit light. When an electric current is passed through an EL material, the atoms are excited, causing them to emit photons. The color may be modified by varying the semiconducting material. EL materials are useful for signage applications, as they are tuneable to virtually any color, providing monochromatic light with a narrow emission peak. The brightness is uniform from any viewing angle. Further, the display screens are usually thin, and have low power consumption. However, high operating voltages (>150 V) are typically required to power EL displays.
Signage may be constructed from liquid crystal displays, which require a backlight, usually from a cathode ray tube. Liquid crystals within the display change their alignment in response to an electric field; this change alters the light transmitted through the device, thus changing the image. For signage applications, liquid crystals provide a lower-energy alternative to fluorescent tubes, and are also safer to dispose of. They may be made into compact and lightweight displays in most shapes and sizes. However, disadvantages include slow response and switching times, which may be unfavorable for dynamic displays, along with a limited viewing angle.
The display technologies currently available for illuminated signage applications provide a variety of formats and colors, which may be more suitable for one particular application over another. Each technology presents its own advantages and disadvantages, however there appears to lack a system that encompasses inexpensiveness and ease of manufacture, with low operating costs, and the availability in a range of colors spanning the entire visible spectrum, in a compact package that may be made to any dimensions desired. In view of the existing technologies, there is a need for a low-power static display that may be produced quickly and inexpensively in a wide range of colors, in any size or shape, which is suitable for use in a range of situations and environments. It is also desirable that the display can be operated safely and poses limited health and safety risks both if damaged and at the end of its lifetime.
Color Tuneability
Displays using a single-colored backlight with a remote medium to tune the emission are often favorable over multiple colored illumination sources, owing to their ease of production and because the electrical circuitry requirements are minimised. LEDs are increasingly replacing incandescent and gas discharge lighting sources for backlighting, since they display superior longevity, lower energy consumption resulting from less energy loss as heat, superior robustness, durability and reliability, and faster switching times. However, with SSL it is difficult to achieve high quality white light and their intensities vary considerably with color. Thus, methods to tune the emission from SSL remotely are frequently employed. Current techniques used to achieve secondary monochromatic light from a backlit source for illuminated signage include color filters and phosphors.
Color filters comprise a white LED backlight with a range of filters to transmit blocks of monochromatic light (FIG. 1). Color filters are often favored as they are inexpensive to produce, however energy losses are high (typically 50-90%), since undesired wavelengths are absorbed by the filter. Thus, the resultant energy output is typically low. In addition, color filters require a broad-spectrum light source; white light is difficult to achieve from LEDs and consequently they are expensive.
Color tuneability may be achieved by combining an LED emitting in the UV or blue region of the electromagnetic (EM) spectrum with a phosphor; phosphorescent materials emit at a longer wavelength than they absorb, as the absorbed radiation undergoes a Stokes shift. Phosphors are traditionally fabricated from transition metal- or rare-earth-doped compounds. Examples include SrSi:Eu2+, MgF2:Mn, InBO3:Eu and SrGaS4:Eu2+, which emit in the red, orange, yellow and green, respectively. Color tuneability is limited by the range of phosphors available. The lifetime efficiencies of phosphors are limited, due to oxidation, crystal lattice degradation, and diffusion processes. Further, they are typically insoluble, making them difficult to process.
QDs, semiconductor nanoparticles of the order of 2-50 nm, can be tuned to emit at any wavelength from the UV to the near-IR region of the electromagnetic spectrum by controlling the particle size, without changing the inherent material.
II-VI chalcogenide semiconductor nanoparticles, such as ZnS, ZnSe, CdS, CdSe and CdTe, have been extensively studied. In particular, CdSe has been widely investigated due to the tuneability of its photoluminescence over the visible range of the EM spectrum. Many reproducible, scalable syntheses are described in the prior art, from a “bottom up” approach, whereby particles are synthesised atom-by-atom, from molecules, to clusters, to particles. Such approaches use “wet chemistry” techniques.
Owing to the toxicity of Cd, it is unfavorable for commercial applications; legislation restricting the use of heavy metals in commercial products is being implemented across the globe, e.g. EU directive 2002/95/EC, “Restriction of the Use of Hazardous Substances in Electronic Equipment” prohibits the sale of new electrical and electronic equipment containing lead, cadmium, mercury, and hexavalent chromium above a specified level. Consequently, attempts to synthesize heavy metal-free quantum dot semiconductors have been explored. One such candidate is the Group III-V semiconductor InP, as well as alloys of this material with other elements. Though the photoluminescence is not typically as narrow as that of Cd-based quantum dots, InP-based semiconducting nanoparticles may be synthesised on a commercial scale with full-width half-maxima (FWHM)<60 nm and photoluminescence quantum yields (PLQY)>90%, and their emission may be tuned across the visible spectrum, from the blue to the red region.
The unique properties of quantum dots arise from their dimensions. As a particle's dimensions decrease, the ratio of the surface to the interior atoms increases; the large surface area-to-volume ratio of nanoparticles results in surface properties having a strong influence on the properties of the material. Further, as the nanoparticle size decreases, the electronic wavefunction becomes confined to increasingly smaller dimensions, such that the properties of the nanoparticle become intermediate between those of the bulk material and individual atoms, a phenomenon known as “quantum confinement”. The band gap becomes larger as the nanoparticle size is reduced, and the nanoparticles develop discrete energy levels, rather than a continuous energy band as observed in bulk semiconductors. Thus, nanoparticles emit at a higher energy than that of the bulk material. Due to Coulombic interactions, quantum dots have higher kinetic energy than their bulk counterparts, thus a narrow band width, and the band gap increases in energy as the particle size decreases.
QDs made from a single semiconducting material passivated by an organic layer on the surface are known as “cores”. Cores tend to have a relatively low quantum efficiency, since electron-hole recombination is a facilitated by defects and dangling bonds on the surface of the nanoparticles, leading to non-radiative emission. Several approaches are used to enhance the quantum efficiency. The first approach is to synthesise a “core-shell” nanoparticle, in which a “shell” layer of a wider band gap material is grown epitaxially on the surface of the core; this serves to eliminate the surface defects and dangling bonds, thus preventing non-radiative emission. Examples of core-shell materials include CdSe/ZnS and InP/ZnS. A second approach is to grow core-multishell, “quantum dot-quantum well”, materials. In this system, a thin layer of a narrow band gap material is grown on the surface of a wide band gap core, then a final layer of the wide band gap material is grown on the surface of the narrower band gap shell. This approach ensures that all photoexcited carriers are confined to the narrower band gap layer, and examples include CdS/HgS/CdS and AlAs/GaAs/AlAs. A third technique is to grow a “graded shell” QD, where a compositionally-graded alloy shell is grown epitaxially on the core surface; this serves to eliminate defects resulting from strain. One such example is CdSe/Cd1-xZnxSe1-ySy.
QD emission may be tuned to higher energies than the band gap of the bulk material by manipulating the particle size. Methods to alter the absorption and emission to lower energies than that of the bulk semiconductor involve doping wide band gap QDs with a transition metal to form “d dots”. In one example, Pradhan and Peng describe the doping of ZnSe with Mn to tune the photoluminescence from 565 nm to 610 nm [N. Pradhan et al., J. Am. Chem. Soc., 2007, 129, 3339].
QD phosphors may be used to down-convert the emission from an inexpensive UV or blue solid-state lighting source. Since QDs may easily be synthesised in any color by manipulating the particle size, the emission may be tuned right across the visible range of the EM spectrum to produce any desired color of display.
In an earlier patent application (US 2010/0123155 A1, filed Nov. 19, 2009, the entire contents of which are incorporated herein by reference), we discuss the preparation of “QD-beads”, in which QDs are encapsulated into microbeads comprising an optically transparent medium; the QD-beads are then embedded in a host LED encapsulation medium. Bead diameters may range from 20 nm to 0.5 mm. QD-beads offer enhanced stability to mechanical and thermal processing relative to “bare” QDs, as well as improved stability to moisture, air, and photo-oxidation, allowing potential for processing in air, which could reduce manufacturing costs. By encapsulating the QDs into beads, they are also protected from the potentially damaging chemical environment of the encapsulation medium. Microbead encapsulation also serves to eliminate the agglomeration that is detrimental to the optical performance of bare QDs as phosphors.
Examples of QD phosphors for display technologies are described in the prior art, however most are based on II-VI and IV-VI semiconductors, such as CdSe and PbSe. Where heavy metal-free QDs are proposed, examples of device fabrication and efficiency have not been discussed.
U.S. Pat. Nos. 7,405,516 B1 and 7,833,076 B1 propose the addition of QDs to the outer shell of a plasma-display device to tune the emission from the gas discharge, however no examples of suitable QDs or methods for their incorporation are offered.
U.S. Pat. No. 7,857,485 B2 discloses an LED display device using LEDs that emit UV or blue light, then a luminescent material, for instance QDs, to tune the LED emission to the desired wavelength. No QD materials are suggested, and no examples of device fabrication using QDs are given.
Patent application US 2009/023183 A1 describes a backlight module comprising a light source and a series of wavelength converters positioned nearby to tune the emission. After passing through a wavelength converter, which may be fabricated from QD material, one portion of converted light is emitted while the remainder is directed towards another wavelength converter. No examples of suitable QD material or their use in device fabrication are disclosed.
A number of patents and patent applications propose the use of QD materials as phosphors. EP 1 758 144 A1, EP 1 775 748 A2, US 2007/0046571 A1 and US 2007/0080640 A1 all describe a plasma display panel device including a QD phosphor layer. EP 1 788 604, U.S. Pat. No. 7,667,233 B2 and US 2007/0117251 A1 disclose a flat lamp plasma display device with a phosphor layer that may be made from QDs. US 2007/0090302 A1 describes a display device including a phosphor layer that may be excited by gas discharge. The phosphor layer may be fabricated from QDs. Though each of these patents makes reference to the use of QDs as phosphors for display technologies, no examples of their use in devices or suitable QD material are provided.
Najjar et al. describe a fluorescent screen and display device with at least one excitation optical beam to excite one or more fluorescent materials on a screen, in patent application US 2006/0221021 A1. The fluorescent material may include phosphors and non-phosphors, such as QDs, though no examples of suitable QDs are specified. Device fabrication incorporating QDs is not exemplified.
The patent application US 2007/0080642 A1 by Son et al. describes a gas discharge display panel with a phosphor layer than may include QDs, but examples of suitable QDs or displays in which they are used are not provided.
Park et al. describe a gas discharge cell with two luminescent layers in patent application US 2007/0241682 A1. The first luminescent layer is composed of a phosphor, while the second may be made of a cathode luminescent or QD materials, though an appropriate QD material is not suggested. No examples of device fabrication with QDs are provided.
The patent application US 2008/019772 by Nam et al. describes a display device comprising a gas discharge tube and red, green and blue phosphors to produce white light. Traditional phosphors or alternatively QD material, which may be printable, may be used. The QD material is not specified, and neither is its printability or incorporation into a display device exemplified.
Bretchnelder et al. propose a light emitting unit, which may comprise an LED and a remote luminescent material, which could include QDs, in patent application publication WO 2011/103204 A2. Examples of suitable QDs and description of their use are not given.
The patent application US 2009/0034230 A1 describes an illumination device that may combine solid state lighting with a wavelength converting material, such as a phosphor and/or QDs, to down-convert the emission. Examples of QD materials and their use are not provided.
Patent application US 2007/0188483 A1 describes display apparatus for outdoor signage. Though it is mentioned that QD material may be used to fabricate an electronic paper-like display, no examples of appropriate QDs or their use in device fabrication are provided.
Two published international patent applications, WO 2010/123809 A2 and WO 2010.123814 A1, describe a display device comprising LEDs with an active layer of quantum wells sandwiched between two doped semiconducting layers, acting as a wavelength converter to down-convert light from the LED source. Despite proposing Group IV: Si or Ge, III-V, or II-VI QDs as appropriate materials, their utilization in display devices is not demonstrated.
The patent EP 2 270 884 A1 describes a display device with a light source and a wavelength modulator, separated by a spacer. The wavelength modulator may be made from an inorganic QD phosphor, although description of its use in the device is not included.
US 2011/0249424 A1 and EP 2 381 495 A2 describe an LED package with an LED backlight and a wavelength converting material. The wavelength converting material may be made from a phosphor and/or QDs. Suitable QDs include Group II-VI and III-VI materials, however no examples are provided for their incorporation into the LED package.
The patent application WO 2010/092362 A2 describes a device having LEDs in close contact with colloidal QDs. CdTe and core-shell CdSe/CdS are given as suitable QD materials, though examples of their use are not provided.
Patent application US 2011/0182056 A1 describes an LED device fabricated from bulk semi-polar or non-polar materials with emission tuned by phosphors, in their The phosphors may be made from QDs, including CdTe, ZnS, ZnSe, ZnTe, and CdSe, to tune the emission with a minimal effect on brightness. No examples are given to demonstrate the use of QDs in devices.
U.S. Pat. No. 8,017,972 B2 and US 2007/0246734 A1 describe a white LED device composed of a UV LED with blue and green phosphors, along with red QDs, which have better luminescence than red phosphors. The QDs are excited by the emission from the blue and green phosphor photoluminescence to alleviate the damaging effect of direct QD exposure to UV light. Group II-VI and III-V QDs are included as suitable materials, though only red CdSe QD synthesis is disclosed.
The patent applications US 2006/0157686, JP 2006/199963 A and US 2011/0121260 A1 describe QD phosphor preparation, with a formulation in which the nanoparticles do not aggregate in the resin, for use in LEDs. It is suggested that the QDs may be mixed with inorganic phosphors. Group II-VI and III-V QD materials are stated as appropriate materials, though only CdSe/CdS core-shell QD synthesis (with 85% quantum yield) is disclosed. Methods for LED preparation are also described.
U.S. Pat. No. 8,030,843 and US 2010/0066775 A1 describe a method for producing QD phosphors for use with UV LEDs. The phosphor material comprises a QD core with an organic capping material and an activator layer. ZnS and ZnO are proposed as appropriate QDs, and their synthesis is included. The synthesis is not by a colloidal method, thus a two-step process is required to synthesise then cap the particles with organic substances such as mercaptosuccinic acid and dithiosquaric acid.
The patent application US 2011/0156575 A1 includes a display device with an illumination unit comprising an LED chip and QD phosphors, and a color filter to enhance the display. It is claimed that red, green and blue QD phosphors may be used, fabricated from both Cd and Cd-free materials. Some data is included to support the use of CdSe/ZnSe QDs.
US 2008/0246017 A1 describes methods for fabricating LED chips with a layer of nanoparticles to tune the emission. It is claimed that Group II-VI, IV-VI, III-V, and I-II-VI QDs may be used. Examples highlighting the color mixing ratios to achieve specific colored emission from QDs emitting at various wavelengths are provides, however only CdSe and PbS QDs are used. No details of QD synthesis are included.
US 2008/0173886 A1 describes a method to produce solid-state lighting using QDs dispersed in acrylates, deposited over the light source to down-convert the emission. It is claimed that Group II-VI, III-V, IV-VI cores, shelled with Group II-IV, III-V, or IV-VI materials or with metals such as Cd, Zn, Hg, Pb, Al, Ga or In, may be used. Methods for QD dispersion and the curing process are described. Examples are included where red CdSe, green CdSe, red and green CdSe, and PbSe have been used in devices, however no QD synthesis is detailed.